A conventional application for an indicator lamp is in an automobile instrument panel. In this application, one indicator lamp is generally provided for each critical vehicle operating parameter. Upon the occurrence of a malfunction condition in a critical operating parameter, a respective indicator lamp will become energized to indicate to the vehicle operator the existence of the malfunction.
In the current generation of automotive technology, a relatively greater number of vehicle operating parameters have become critical or significant to the proper operation of the vehicle. A specific example is a case of vehicles equipped with an electronic fuel injection (EFI) system. As EFI system will control fuel injection characteristics on the basis of several significant engine operating parameters, e.g. manifold air pressure, exhaust gas recirculation, air temperature, coolant temperature, O.sub.2, RPM and speed command are all types of parameters used by various EFI systems. With this expanded class of parameters comes the desire to include the capability to notify the vehicle operator of a malfunction in the sensing of any one of the conditions which may adversely affect EFI control. Accordingly, there is a commensurate need to expand the number of indicator lamps or similar type display devices on the instrument panel.
However, there is for practical purposes a limitation on the number of indicator lamps or display devices that can be driven by the control logic of an EFI system. More specifically, most practical EFI systems are presently being designed with microprocessors implementing the control logic. A commerical microprocessor has a limited number of input/output (I/O) terminals available for assignment to driving indicator lamps. Moreover, in addition to driving the lamp, it is important that the control logic include the capability to detect a short or similar type fault condition in the lamp circuitry itself to disable energization of the lamp if that should occur. This latter capability places a further demand on the I/O resources of the microprocessor.
An important objective of the present invention is to therefore provide the capability to both control energization of an indicator lamp and detect a short or fault condition in the lamp circuitry with a minimal demand on available microprocessor I/O terminals. It is a specific objective to provide a driver circuit and associated method of operation that requires only one microprocessor I/O terminal for each indicator lamp or similar type display device.